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Old 5th March 2004, 07:49 PM   #11
sdoom is offline sdoom  Germany
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Hi guys, right now this thing is starting to p. me off,


OK, I got the MOSFET , the guy said I can replace a IRFZ44 with a IRFZ44N, they are the same.

I replaced all of them and also the caps.

Turned on the amp and now I have this ugly noise . Canīt determine from which transformer it comes, I assume both as I plugged out one fuse to have one side run seperately and the noise was still there. And also now I have a hige idle current on the 12V side which I didnīt have with the IRFZ48N. It starts with about 2A , then the noise gets louder and the current draw goes up to 5A and by that time I shut the AMP off.

I tried to measure some frequencies. The TL494 has to outputs and a refernece voltage output. The ref-voltage is 5V and itīs stable.

those are the frequencies I could get from the TL494-pins:

Pin 8 (Collector1) : running from 10kHz to 99 kHz and back
Pin 9Emitter1) 29 -30 kHz no big change
Pin 10(Emitter2): 25-30 kHz no big change
Pin 11(Collector2): 80kHz at start , when noise gets loud it goes to a 120kHz
Pin 14: 5V stable ref out voltage

The AC-voltage across the capacitors is about 1V , but with 120kHz no wonder they blow in smoke.

There are 4 transistors (2SA1266) that drive the 4 Mosfet-packs. Each MOSFET is connected with a 100Ohm (sorry I mentioned 150 earlier but that was wrong) and there is also a gnd-resistor that has 1kOhm.


Sorry guys for beeing annoying. But if I get a scope where would I have to look first. I checked all parts in the power supply and they are all ok. If it would help I could make a curcuit diagramm of this thing ?

How can I check on the transformer coils, I assume that Eva is right and I cant use the transformer . But the company that made the Amp is out off business and I cant order the exact one. As far as windings concern they are absolut identical, so it must be the core.
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Old 5th March 2004, 11:36 PM   #12
mwh-eng is offline mwh-eng  United States
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Quote:
Originally posted by sdoom
Hi guys, right now this thing is starting to p. me off,


OK, I got the MOSFET , the guy said I can replace a IRFZ44 with a IRFZ44N, they are the same.

I replaced all of them and also the caps.

Turned on the amp and now I have this ugly noise . Canīt determine from which transformer it comes, I assume both as I plugged out one fuse to have one side run seperately and the noise was still there. And also now I have a hige idle current on the 12V side which I didnīt have with the IRFZ48N. It starts with about 2A , then the noise gets louder and the current draw goes up to 5A and by that time I shut the AMP off.

I tried to measure some frequencies. The TL494 has to outputs and a refernece voltage output. The ref-voltage is 5V and itīs stable.

those are the frequencies I could get from the TL494-pins:

Pin 8 (Collector1) : running from 10kHz to 99 kHz and back
Pin 9Emitter1) 29 -30 kHz no big change
Pin 10(Emitter2): 25-30 kHz no big change
Pin 11(Collector2): 80kHz at start , when noise gets loud it goes to a 120kHz
Pin 14: 5V stable ref out voltage

The AC-voltage across the capacitors is about 1V , but with 120kHz no wonder they blow in smoke.

There are 4 transistors (2SA1266) that drive the 4 Mosfet-packs. Each MOSFET is connected with a 100Ohm (sorry I mentioned 150 earlier but that was wrong) and there is also a gnd-resistor that has 1kOhm.


Sorry guys for beeing annoying. But if I get a scope where would I have to look first. I checked all parts in the power supply and they are all ok. If it would help I could make a curcuit diagramm of this thing ?

How can I check on the transformer coils, I assume that Eva is right and I cant use the transformer . But the company that made the Amp is out off business and I cant order the exact one. As far as windings concern they are absolut identical, so it must be the core.

Grim!

The IRF44 and IRF44N are similar, but have differences. Download the PDFs on each and compare them to each other. Not much difference, but some.

Download the data sheet and an application note for the TL494. The dead time control has a separate input from the RC oscillator inputs. I've not used this IC before, but you should see a fixed frequency on either the R or C input pin. Without your schematic for the DC to DC converter it's hard to speculate, but I would think that either pin 8 or 9 and either pin 10 or 11 should have a fixed frequency. Something is definitely wrong with the converter section.

I think you need to do some troubleshooting with just the TL494 powered, without the Mosfets or Transformers in the circuit. Need to make sure everything is alright with the PWM first. If you can scan or draw us a schematic of just the converter or even a partial schematic it may help us to see your problem.

One method to get an idea if the transformer core has similar permeability of your other original transformer is to do a resonant frequency test and calculation. You will need a variable frequency sinewave generator for this method. With your transformer disconnected and the secondary windings open circuit. Connect your signal generator (Red lead) to one lead of the primary and connect a .1uf to 1uf capacitor to the other lead. Then connect a 1K ohm resistor to the other lead of the capacitor. Connect the other resistor lead to the return or ground connection of the signal generator (Black lead). Connect your Oscope or a DVM (one that can measure up to 30 KHz or so) across the resistor. Adjust the signal generator and look for a peak in the voltage across the resistor. You should find a resonant peak where the voltage is maximum. Either side of this resonant point the voltage across the resistor should drop. You want to start at a low frequency and adjust upwards to avoid finding a resonance on a harmonic of the fundamental. You may have to try different values of capacitance to find a resonance within the range of your signal generator and DVM. Once and if you find the lowest resonant point. Use this frequency to calculate the inductance of the primary:

Lprimary = 1/((2*pi*f)^2)*C

pi = 3.414
f = resonant frequency
C = capacitor value
Lprimary = primary inductance of transformer

Now repeat the experiment with your other transformer(s). See if the inductances are within 10 percent of each other. If the inductance is far different then the replacement core probably has a different permeability (made from a different material) and probably will not work in this application.

Again, try to isolate the problem further without having the Mosfets or transformer in the circuit. We need to make sure the TL494 is working correctly first. Try to get us a schematic.

I can imagine your frustration, but you stand a chance of learning something important. Try to be patient.
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Old 6th March 2004, 05:24 AM   #13
sdoom is offline sdoom  Germany
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Hi

I assumed you would say that and I really appreciate your help.
It will take me about a week to get the scope and I hope I can have the schematics ready by the mid of next week.


What I find strange about this whole thing is when I first started to do some work on the Amp was that I only used 1 FET per coil and there was no noise and I had the whole thing running for at least 30 minutes without a heatsink and this thing was working. I do have some understanding of electronics but I totally agree I can learn a lot from you guys when it comes to PWM-power supplies.

I assume the manufacturer already had some problem with this thing as they had connected an additional small 10ĩF capacitor across a switching transistor.

I will also make pictures and post them. Concerning the reason for the amp to malfunction I think one of the windings of transformer No.2 had an isolation problem and caused a short , that was how it looked like. The core wasnīt broken completely but there was a big edge broken out.

thx again

stephan
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Old 6th March 2004, 05:00 PM   #14
mwh-eng is offline mwh-eng  United States
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Quote:
Originally posted by sdoom


What I find strange about this whole thing is when I first started to do some work on the Amp was that I only used 1 FET per coil and there was no noise and I had the whole thing running for at least 30 minutes without a heatsink and this thing was working. I do have some understanding of electronics but I totally agree I can learn a lot from you guys when it comes to PWM-power supplies.

I assume the manufacturer already had some problem with this thing as they had connected an additional small 10ĩF capacitor across a switching transistor.

I will also make pictures and post them. Concerning the reason for the amp to malfunction I think one of the windings of transformer No.2 had an isolation problem and caused a short , that was how it looked like. The core wasnīt broken completely but there was a big edge broken out.

I apologize for treating you like someone with little experience. Some people can have a lot of experience and education, and still have trouble understanding magnetics and switching power supplies. This is a subject most colleges and technical schools don't teach and must be learned the hard way, at least in my experience. I have many years of experience with power conversion products and still get stumped sometimes. This is not trivial stuff.

From what you are saying, it sounds like this product had or maybe still has some design problems. Maybe the design is so close to the edge, any little component change causes instability. A marginal design could have caused the original failure. I've had my patience tested many times by SMPS stability and noise problems.

Since the manufacturer connected that 10 uf cap across the switching transistor, they may have had some noise induced stability problem(s). To me, these are the most difficult ones to identify and solve. I suspect that I will learn from this experience.

I have an old scope and need a newer one. Ebay has some good deals or at least they did a few months ago. Those used Tektronix Scopes such as the 465 and 475 are great old scopes. I've seen some refurbished ones with warranty go for around $200.00 or a little more.

See if you can find whether or not this converter has feedback for regulation. Usually they just use a push-pull topology with a transformer and no output inductors. This one of yours sounds more complicated than that, however. If it does not have feedback for regulation, it may be easier to figure out. If you are reverse engineering it to make a schematic, that is difficult.

Since you will probably have to re-design this converter and make more modifications, you will likely need a scope, unfortunately.

Keep us posted and good luck!

Mark
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Old 6th March 2004, 08:51 PM   #15
sdoom is offline sdoom  Germany
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@ mwh-eng

please don`t apologize , I am really really thankful for your help. There are not many people around who really have knowledge about the stuff like you do, and I wanna learn,too ! And your description on how to test the cores is really good.

See, I know in basics how a PWM-supply is working and how I can test a MOSFET if it is working or not, but I am not into the stuff like you are, so far, I fixed about 30-40 car amps (and they still work .. ..) A lot of work (not only in electronics) can be done without the exact knowledge of how it works, but as in this case in German you īd say
"jetzt geht es an das Eingemachte" meaning now you have to take out the last preserves"

And yes I had an electronics job training (lasted 3 years) . But that is a long time ago and we did not even had MOSFET in that job training. Also now I attend evening school again. I passed almost 3 years now and there is one to come, but it all goes into data transfer and not into power electronics.
Today I was helping my brother who was moving. Besides boxes and furniture I havenīt seen anything today and I īll go to bed pretty soon . Maybe tomorrow Iīll find time to get the schematics for you. As far as I have overlooked it, there arenīt too many parts.

Iīm not sure if the whole assembly is really running on the upper limit, at least what I have seen on car Amp power supplies, in most standard car amps they use 4 IRFZ44 with a 40 Amp fuse.


And as far as I understand the TL494 it uses a fixed frequency for switching and only changes the ON/OFF-time depending on the load. So in my understanding there must be a feedback . Donīt they use the reference voltage generator for that and compare the output voltage to it ? There are a few Z-diodes where they probably "waste " the voltage on and take "whatīs left off it" and compare it to the refernece voltage ? Or am I totally wrong on understanding this thing? Because, what I have overlooked, not all Z-diodes are for generating the +/-15V supply voltage for the OP-Amps.

thx again
stephan
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Old 6th March 2004, 08:58 PM   #16
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Hi,

I didn't read the whole thread so maybe this has been covered and in that case forget it. But, there could be higher current spikes from the primary caps through the input chokes, which means when this higher (or faster) curent switches off, there is (much) higher voltage across the caps, causing them to overheat and fail. Try higher voltage caps, even with a bit lower capacitance, I bet that solves it.

Jan Didden
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Old 6th March 2004, 09:50 PM   #17
sdoom is offline sdoom  Germany
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@ janneman

well right now I have the MOSFET installed that are supposed to be working in there and now the whole thing is totally instable. The switching frequency changes rapidly from 15kHz up to 120kHz causing a 5A (12V side) idle current. Itīs definetily not he secondary side of the supply curcuit. The diodes are ok and the caps on the secondary side ,too. With no load the idle current should be around 500mA to 1A and there should not be any noise. Problem is I donīt have a scope yet so I cannot really see what is goinf on in there. With the IRFZ48N I had at least 10 min. of clear signals and the output voltage was +/-52V stable. When the caps on the input side have died the current draw went way up and I had the bad noise. The manuf. already had a small cap soldered across a switching transistor so I assume the stock layout is vulnerable for stress/instability. But well, the amp was fine for at least 6 years working time.


The original Caps had 25V , I already installed 40V. With a 63V capacitor I am kind of running out of space here )
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Old 6th March 2004, 11:15 PM   #18
Eva is offline Eva  Spain
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Measure the 'clock' or 'CT' frequencies on the TL494. If the IC has constant power supply but these frequencies are not constant, then the TL494 may be bad. Look also for false solder joints on the TL494 and around it
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Old 8th March 2004, 02:23 AM   #19
mwh-eng is offline mwh-eng  United States
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Quote:
Originally posted by sdoom
And yes I had an electronics job training (lasted 3 years) . But that is a long time ago and we did not even had MOSFET in that job training. Also now I attend evening school again. I passed almost 3 years now and there is one to come, but it all goes into data transfer and not into power electronics.
Most engineers/technicians don't have the opportunity to attend a college that offers courses in power electronics. As time passes more and more American universities are offering courses in power electronics, however. Most of us, like you and I must learn this difficult subject the hard way through experience. I was fortunate enough to work with others that had good power electronics experience and I owe much to them.


Quote:
Iīm not sure if the whole assembly is really running on the upper limit, at least what I have seen on car Amp power supplies, in most standard car amps they use 4 IRFZ44 with a 40 Amp fuse.[/B]
Some consumer products are designed such that they are pushed too close to the limits, causing them to have high failure rates. An automobile can be a hostle environment. Think about the inside temperature after sitting in the hot sun for a few hours. Depending on the lattitude, it can be close to 150 F. Jump in the car and turn the audio power amp on full blast and you could have a heat related failure. It's best to turn off all audio and other unnecessary electronics until the inside of an automobile has cooled off to the ambient outside temperature. Not much cool off in a place like Phoenix, Arizona at 122 F summer time temperature. The heat sink of a power amp has some thermal mass and an associated thermal time constant. Of course, don't tell your customers about this because you won't have as many amps to repair. :^)


Quote:
And as far as I understand the TL494 it uses a fixed frequency for switching and only changes the ON/OFF-time depending on the load. So in my understanding there must be a feedback . Donīt they use the reference voltage generator for that and compare the output voltage to it ? There are a few Z-diodes where they probably "waste " the voltage on and take "whatīs left off it" and compare it to the refernece voltage ? Or am I totally wrong on understanding this thing? Because, what I have overlooked, not all Z-diodes are for generating the +/-15V supply voltage for the OP-Amps.
[/B]

Most auto audio power amps have buck derived DC/DC converters, or straight DC/DC squarewave converters without an output inductor. The DC/DC converter with output inductor of the proper size is usually a buck derived converter. This converter may or may not have feedback. On a buck converter with feedback the duty cycle changes with load only to make up for rectifier voltage drops, I*R drops in the transformer and output inductor, RDSon*I drop, and other I*R drops. The transfer function of a buck regulator is Vout = Vin*Dutycycle. Ideally it's not load dependent; however, it is not ideal and has losses that must be made up for if a regulated output is maintained.

The transformer in a DC/DC converter cannot have a 100 percent power transfer/coupling. Coupling is related to leakage inductances. Voltage drops across leakage inductances = Lleak*(di/dt).

Since Vout = Vin*D.C.; Vout varies in direct proportion to Vin. The battery voltage in an auto typically varies only with temperature (engine running), because a lead acid battery voltage is somewhat temperature dependent. If the DC/DC converter has a 1:4 turns ratio, then a 1 volt change on the battery yields a 4 volt change at the converter's transformer secondary.

Adding the transformer turns ratio to the voltage transfer function of the buck:

Vout = Turns_Ratio*Vin*D.C.


In the case of a DC/DC squarewave converter without an output inductor, there is usually no feedback for regulation. They work best running at high duty cycle. The secondary of the transformer is rectified right into capacitors without an inductor. The output voltage will vary due to rectifier voltage drop variations, I*R drops, and input voltage variations. These converters tend to be simple but noisy.

Note: Since I've not been active in SMPS design or development work in nearly 15 years, there could be errors in my explanations. I need to find a good practical textbook to suggest. There are many books on switch-mode power supplies, but most are not complete, or are out of date with modern techniques. A lot can be learned from application notes. Unitrode Corporation made some of the best ICs for power conversion. Their seminars were great and their application notes are some of the best. Texas Instruments bought them out a few years ago. Check out all these power conversion products starting at www.ti.com and follow the Power Management Link.

The following is a very good textbook on power electronics, but expensive.

FUNDAMENTALS OF POWER ELECTRONICS, 2nd Ed.
by Erickson, Robert & Maksimovic, Dragan

Contents: Principles of Steady State Converter Analysis; Steady-State Equivalent Circuit Modeling, Losses, & Efficiency; Switch Realization; Discontinuous Conduction Mode; Converter Circuits; AC Equivalent Circuit Modeling; Converter Transfer Functions; Controller Design; Input Filter Design; AC & DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode; Current Programmed Control; Basic Magnetics Theory; Inductor Design; Transformer Design; Power & Harmonics in Nonsinusoidal Systems; Line-Commutated Rectifiers; Pulse-Width Modulated Rectifiers; Resonant Conversion; Soft Switching.
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Old 9th March 2004, 11:52 AM   #20
sdoom is offline sdoom  Germany
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hi folks

Sorry for not writing sooner. Iīm doing the schematics right now and hope to be done with it tonight. And I will get a scope tomorrow, too.

If the only problem was the transformer core #2 could I use the old one and rewire it? There is a "big" edge missing but this core is still complete

I will make pictures of everything by the time the schematics are done so you will have a complete package.

Thx again
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